1. Field of the Invention
The present invention relates to a reflection liquid crystal display, and in particular relates to a liquid crystal display in that visibility is improved by imparting optical diffusion to a liquid crystal cell.
2. Description of the Related Art
The reflection liquid crystal display is a liquid crystal display using only ambient light such as sunlight and illumination as the illumination light, and it is widely used in portable information terminals and the like requiring low electric consumption. A semi-permeable liquid crystal display is another example in that under a circumstance where ambient light cannot be sufficiently obtained, it is operated in a transparent mode by turning on a backlight while when ambient light can be sufficiently obtained, it is operated in a reflection mode without turning on the backlight, so that this display is widely used in portable electronic instruments such as mobile phones and notebook personal computers.
The reflection liquid crystal display is required to have bright display performance. In order to achieve this display performance, it is important to control diffusion performance to incident light from outside which is then reflected by the inside of the reflection liquid crystal display and emitted again outside. Therefore, in the reflection liquid crystal display, in order to impart the performance in that incident light from every angular direction can be reflected in a display direction (to an observer), a system imparting diffusion performance to a reflection plate provided inside or outside the liquid crystal display or a front diffusion system in that a diffusion layer is formed inside the liquid crystal display so that light is scattered during passing through the diffusion layer is incorporated in the reflection liquid crystal display.
FIG. 9 is a side sectional view of an example of a conventional reflection liquid crystal display having a reflection plate with diffusion performance disposed in a liquid crystal panel. This reflection liquid crystal display includes a light-transmissive opposing substrate 101, a liquid crystal layer 110, and a light-reflective element substrate 102 sequentially arranged in a light-incidence direction, and the element substrate 102 is provided with a reflection diffusion band reflecting and scattering light Q passed through the opposing substrate 101. The diffusion band is made of a reflection plate 130 having a high-reflectance metallic film 122 with corrugated portions 122a formed on the surface and an insulating layer 128 as an under layer. A region per one pixel of the reflection plate 130 is divided into two regions of a region A with strongly directional reflection characteristics and a region B with strongly diffusive reflection characteristics, and each region has a irregular surface formed with an average inclination angle different from each other.
In addition, this reflection liquid crystal display can also be used as the semi-permeable type by reducing the thickness of the high-reflectance metallic film 122 or by forming micro-pores for transmission.
FIG. 10 is a drawing showing reflection characteristics of the reflection plate provided in this reflection liquid crystal display. In the drawing, a curve A is a reflection characteristic profile in the region A of FIG. 9; a curve B is a reflection characteristic profile in the region B of FIG. 9; and a curve C is a reflection characteristic profile in the entire one pixel. The reflection characteristics are dependency of an emitting angle of reflection light measured by fixing a white light source in a normal direction relative to the surface of the reflection plate and rotating a detector for measuring reflection light intensity.
The curves (A) and (B) are profiles with Gaussian distribution about a regular reflection angle of incidence light as a center. The distribution width of each curve is reflected in reflection characteristics of each of the regions (A) and (B). That is, the half-band width of the profile of the reflection characteristics (B) is larger than that of the reflection characteristics (A).
The curve (C) of the profile showing the final reflection characteristics of one pixel exhibits Gaussian distribution about a regular reflection angle of incidence light as a center in the same way as in the curves (A) and (B), and the half-band width of this profile shows an average of the entire one pixel.
In the case where a liquid crystal display is assembled in a device used by inclining a display surface like mobile phones and notebook computers, the device is generally viewed at an angle close to a normal direction H relative to the display surface, as shown in FIG. 11. In general, an angle θ1 defined by a principal visual direction α when an observer (user) views the display surface (screen) and the normal direction H is in a range between 0° and 20°.
FIG. 11 is an explanatory view of a service condition of a mobile phone having a liquid crystal display 100 provided in a body 105. Referring to FIG. 11, numeral H denotes a normal line relative to the display 100; numeral Q denotes incident light; and numeral ω0 denotes an incident angle (30°, for example). Also, R1 represents reflection light (regular reflection) when the incident angle ω0 equals to a reflection angle ω; R2 represents reflection light when the reflection angle ω is smaller than the incident angle ω0; and R3 represents reflection light when the reflection angle ω is larger than the incident angle ω0.
As is understood from the drawing, an observing point Ob of the observer is generally directed to the reflection light R2 close to the normal direction H. More specifically, the observing points Ob are concentrated within a range of 10° from the normal direction H. Whereas the reflection light rays R1 and R3 are upward directed to the display surface from the below so that they are difficult to see. Therefore, in view of observer's conveniences, it is required that while a wide angle of visibility is maintained, the reflection light R2, in which the reflection angle is smaller than the regular reflection angle, is increased.
However, in the conventional reflection liquid crystal display shown in FIG. 9, the reflecting range of incident light is increased. That is, although the optical diffusion is achieved, most part of incident light is reflected in the regular reflection direction and in the direction R1 close to the regular reflection (exhibiting reflection characteristics with the Gaussian distribution), so that although the display viewed in the regular reflection direction and directions close to the direction is bright, the display viewed in the other directions is dark.
Therefore, when the display surface of a mobile phone or the like having the conventional reflection liquid crystal display provided in the display unit is viewed, since the observing points of an observer are generally concentrated in a direction close to the normal direction H as mentioned above, the display is dark and for viewing the bright display, the display must be viewed in the regular reflection direction R1 and its close directions, so that the display surface must be upward viewed from the below, resulting in difficulty in viewing the display, as described above.
Also, in the conventional reflection liquid crystal display shown in FIG. 9, the reflection plate 130 having a corrugated reflection surface is used; however, although fine corrugated reflection surface improves the controllability of reflection light, the spectrum is prone to be produced under strong sunlight so that there has been a problem that a rainbow pattern is viewed on the display screen, damaging visibility.
When a front light type prism light-guide plate is arranged in the display adjacent to an observer as a light source, the spectrum is prone to be produced so that there has also been a problem that a rainbow pattern is viewed on the display screen, damaging visibility.